1. Technical Field
This invention relates generally to a method and apparatus for measuring the thickness of a pipe wall; and more particularly to a method and apparatus for measuring the thickness of a pipe wall based at least partly on ultrasonic reflections.
2. Description of Related Art
Processing ultrasonic reflections for the purpose of wall thickness measurements can be particularly challenging when the surfaces being interrogated are not uniform. Typically in such an application an ultrasonic pulse of a particular frequency is injected into the material to be measured from one side. A portion of the ultrasonic pulse will bounce back and forth within the material due to impedance mismatches at the material boundaries. These pulses can be detected and the time between each pulse measured to give the thickness of the material. However, detecting the correct pulses can be difficult in situations where multiple reflections occur such as when the surfaces of the material being measured are irregular. FIG. 1 shows the two potential conditions. FIG. 1a shows a material having a regular surface that provides clear pulses shown in FIG. 1a′ that are easy to detect, FIG. 1b shows a material having an irregular inner surface that provides multiple reflections which are more complicated to detect as shown in FIG. 1b′. 
Traditional peak detection or correlation detection techniques can have a very difficult time detecting the pulses accurately in the presence of the multiple reflections, each of which has a slightly different time of arrival. Additionally, Fourier decomposition and filtering analysis techniques have a difficult time helping to resolve the correct peaks since each reflection has the same frequency components as the desired pulses, resulting in limited filtering gain by only reducing other system noise.
FIG. 2 shows a typical detected signal when an ultrasonic pulse is injected into a pipe with a slightly rough inner surface. The initial pulses can be seen clearly however the multiple reflections quickly engulf the main reflections and toward the end it is difficult to distinguish between the two. As the inner surface of the pipe gets rougher or more irregular, the main pulse signals will decrease drastically in size and it will be difficult to distinguish even the first several pulses.